Why do I only get 4.3GB of data on a 4.7GB DVD?

There's an unfortunate confusion of units of measurement in the DVD world. For example, a single-layer DVD holds 4.7 billion bytes (G bytes), not 4.7 gigabytes (GB). It only holds 4.37 gigabytes. Likewise, a double-sided, dual-layer DVD holds only 15.90 gigabytes, which is 17 billion bytes.

The problem is that the SI prefixes "kilo," "mega," and "giga" normally represent multiples of 1000 (10^3, 10^6, and 10^9), but when used in the computer world to measure bytes they generally represent multiples of 1024 (2^10, 2^20, and 2^30). Both Windows and Mac OS list volume capacities in "true" megabytes and gigabytes, not millions and billions of bytes

Most DVD figures are based on multiples of 1000, in spite of using notation such as GB and KB that traditionally have been based on 1024. The "G bytes" notation does seem to consistently refer to 10^9. The closest I have been able to get to an unambiguous notation is to use "kilobytes" for 1024 bytes, "megabytes" for 1,048,576 bytes, "gigabytes" for 1,073,741,824 bytes, and "BB" for 1,000,000,000 bytes.